A widely known method of forming a plating film on a steel strip surface may include a hot dip method of immersing a steel strip into a molten plating metal.
In a continuous hot dip zinc plating of this type, a steel strip is carried out with a thermal treatment in a pre-heating furnace and a surface cleansing treatment, subsequently immersed into a molten zinc bath so as to form a plating film thereon, drawn out of the bath, adjusted in an amount of plating adhesion by squeezing gases, and performed with a surface treatment by means of a galvannel or the like.
Thus obtained molten metal plated steel sheets have more beautiful, and are excellent in an anti-corrosive property, and hence such steel sheets are widely used.
There have arisen, however, many problems invloved in the conventional hot dip zinc plating methods because of using a plating bath. In recent years, it has been required that the steel strip be more uniform, smoother and finer on the surface than ever before, particularly in the application of external sheets to automobiles, house electric appliances and so on.
Besides, there are increasingly demands for new products of thickness-differential plating or one-side plating in terms of types thereof. For this reason, there appear problems about quality of the plated steel strip based on the conventional hot dip method or peculiar to the process itself. Some of the problems will be given as follows.
(1) Fe elutes from the steel strip surface into the plating bath, or so-called dross is much generated due to oxidation of a plating metal. The dross has to be scooped up and removed, but the metal is lost thereby other than adhered to the steel strip. PA1 (2) The dross is produced in the plating bath, or dusts of bricks composing a pot are mixed in the bath. Thus, impurities are easily mixed in the plating bath and adhered to the steel strip, thereby degrading the appearance thereof. PA1 (3) Plating matrix components to be put into the plating bath, the components adhered to the strip, and trace elements contained in the components to be exhausted outwardly of the plating bath are different from each other. It is therefore difficult to control them to be plating bath components containing necessary elements as desired. PA1 (4) It is required that mechanical members such as rolls for passing the steel strip, arms for supporting the rolls and bearings are immersed into the metal plating bath having a high temperature and high anti-corrosive property.
The above mentioned matters result in inferior plating adhesion and poor alloying of a galvannel material, and also various plating defects.
This causes problems in which those mechanical members are corroded, the concomitant dross is generated therewith, and the plating surface is degraded in appearance due to the corrosion caused in the roll surfaces in the bath.
In addition, the time for the operation stop is needed for periodically repairing and replacing the corroded or damaged parts of the mechanical members, and productive abilities of the apparatus cannot be effectively exhibited to a possible maximum.
(5) Grooves chased in the rolls are easily translated onto the plating surface by using the steel strip passing rolls in the plating bath, thereby degrading the appearance thereof.
(6) The workers bear burdens and are exposed to dangerous situations when removing bottom drosses accumulated on the lower part of the bath and top drosses accumulated on the bath surface, initially passing the steel strip into the bath, repairing the rolls in the plating bath, and performing the operations at high temperatures around the large plating bath.
(7) Since one kind of platings is operated per one pot, and when carrying out different plating processes, it is necessary to replace the bath by scooping, or prepare beforehand the pot in which a heterogeneous plating metal is molten, followed by shifting the pot.
(8) In a case of producing a double-side plating material and a one-side plating material by a single equipment, the plating equipment in the pot unit is needed to be changed. This requires a good deal of time and labors in addition to burdens in the equipment for this purpose.
(9) It is difficult to process special platings such as double-side heterogeneous plating, multi-layered plating, double-side thickness differential plating or the like.
In contrast with the above mentioned conventional hot dip method, there is proposed a hot dip method in Japanese Patent Specification Laid-Open No. 61-207555, where a nozzel is made close to a surface of a travelling steel strip, a molten metal supplied from a tank is sucked by the nozzle by a wet adhesivity between the molten metal and the steel strip surface, and the molten metal is then adhered to the steel strip.
This method makes use of a technique of coating a paint having a high viscosity. The method feeds the molten metal from the tank to the nozzle, in which an amount of plating adhesion is controlled in accordance with a head pressure of the molten metal tank. As a result, variations in a bath level within the tank appear as scattering in amount of plating adhesion. This makes bad an accuracy relative to the amount of plating adhesion. In any case, a molten metal tank similar to the dipping plating bath is required, so that the above mentioned various problems appear.
As discussed above, the conventional hot dip method brings various problems.
In view of such circumstances, it is an object of the invention to provide a new plating method which may continuously plate a metal plate without employing the conventional molten metal bath, and control an amount of palting adhesion with a high accuracy.